The invention relates to an apparatus for controlling the emission of an electronic flash which is used in a photographic camera of the type in which the photometry of reflection from a film surface is effected, and more particularly, to such apparatus which may be used in combination with a single lens reflex camera of the type effecting photometry of reflection from a film surface in order to control the emission of flashlight from a TTL (through-the-lens) "automatic electronic flash" used in combination therewith.
A single lens reflex camera employing a focal plane shutter and arranged to effect the photometry of reflection from a film surface is known in the prior art, which includes double switch means comprising a first switch which is changed from its off to its on condition in synchronism with the completion of running of a first blind of the shutter, and a second switch which is changed from its on to its off condition in synchronism with the initiation of running of a second blind, the first and the second switches being connected in series and the X-contacts of an electronic flash being closed only when both switches are turned on. The double switch means disables the X-contacts for a shutter period which is less than the time required to achieve a full opening of the focal plane shutter, by detecting the fully open condition thereof, and thus is operative to provide a control on the basis of an actual result of shutter running. Accordingly, when an electronic flash is mounted on a single lens reflex camera incorporating such double switch means to enable a TTL "automatic electronic flash" photography, it is difficult to predict if the emission of flashlight from the electronic flash occurs reliably before the shutter release takes place.
It is possible to determine if the emission of flashlight from an electronic flash occurs in all probabilities, by performing an APEX calculation of TTL photometric value beforehand to see if a resulting shutter period is less than the time required to achieve a full opening of a focal plane shutter. However, with this approach, if the shutter period displayed in accordance with the APEX calculation is close to the time required to achieve a full opening of the shutter (for example, 1/60 second), a precision in the display presents a problem. Also, the displayed value does not always match an actual period determined on the basis of the photometry of a film surface, with the result that the occurrence of the emission of flashlight from an electronic flash cannot be determined unless the shutter release actually takes place. Thus it will be seen that an emission control may not be coincident with a prior recognition of a photographer.